1. Technical Field
The invention relates to an AC amplifier and, more particularly, to an AC amplifier with automatic DC compensation.
2. Description of the Related Art
Current in the primary winding of an output transformer of a push-pull like AC power amplifier is not desirable, since it severely degrades the performance of the amplifier. This DC current can develop as a result of various circuit imbalances.
Referring to FIG. 1, a block diagram of a known switching amplifier 10 which is used as an uninterruptable power supply to provide standard 120V, 60Hz system output power at an output 11 is shown. A precision sine wave oscillator 12 generates a sixty Hz reference sine wave signal as the input of a wave shaping feedback system formed by the remaining circuitry. An error amplifier 14 compares the reference sine wave signal at an input 16 of the error amplifier with the system output applied to an input 18. The system output at input 18 is taken from the secondary winding 20 of a transformer T1 and applied to input 18 through means of a feedback line 19. The error amplifier 14 generates an error signal on its output 22 that is either positive or negative with respect to a preselected reference voltage from a reference voltage source 24 depending on the relation of the magnitude of the system output signal at input 18 with the magnitude of the wave shaping feedback system input at input 16. The magnitude of the error signal on output 22 thus depends on the magnitude of the error between the system output signal at input 18 and the system input signal at input 16 of the differential error amplifier 14.
The error signal an output 22 is applied to a conventional DC controlled pulse width modulator 26. The pulse width modulator produces two drive signals, A and B, on modulator outputs 28 and 30, respectively. These A and B drive signals are amplified by a pair of amplifiers 32 and 34, and the amplified drive signals are then applied to drive a pair of switching transistors Q1 and Q2.
When the error signal is positive with respect to the reference from the reference voltage source 24, signal A on output 28 is active. A stream of high frequency pulses is applied to turn Q1 on and off at a high frequency on the order of sixty KHz. When transistor Q1 is intermittently turned on, one side 36 of the primary winding of transformer T1 is intermittently driven to ground relative to DC voltage VS applied to a center tap 40. This produces a positive polarity phase of the AC output signal at output 11. The magnitude of the error signal controls the duty cycle of these pulses: A large error signal produces a large duty cycle, as high as 95 percent, while a small error signal produces a small duty cycle, as small as five percent. In a complementary and similar manner, a negative error signal causes signal B on output 30 to control the duty cycle of the pulse train applied to switching transmitter Q2. When Q2 is turned on, the other side 38 of the primary winding is connected to ground and the negative polarity phase of the AC output signal is generated.
The output transformer T1 and a filter capacitor C1 low pass filter the alternating streams of positive and negative high frequency pulses applied to the primary winding to produce therefrom a smooth AC sinusoidal output signal. The magnitude of the AC output voltage is proportional to the magnitude of the duty cycle of the drive signal applied across the primary winding.
A major problem with the AC amplifier of FIG. 1 is the introduction of DC current into the output transformer. The DC current may arise from several different imbalances in the system, but the most important factor controlling the DC current is the average duty cycle of the output transistors Q1 and Q2. If the average duty cycles of the drive signals A and B are equal, then the current due to this factor will be zero. If the average duty cycles are not equal, a net DC voltage will be applied to the output transformer 20, resulting in a net DC current.
It is known to have local feedback signal applied to the error amplifier 14 through means of a differential amplifier 46 having one input 42 connected to the output of error amplifier 14 and another input 44 connected to the reference voltage source 24. This tends to cause the average value of the error signal on output 22 to equal the reference voltage of source 24 since it tends to equalize the duty cycles. However, because of various system imperfections and the static nature of the feedback, a net DC imbalance can still result.